Diarrhea remains a common cause of childhood morbidity and morality in developing countries yet in the majority of episodes, the etiology cannot be determined. We will use new diagnostic techniques to investigate the natural history of rotavirus, astrovirus, enteric adenovirus, and the Norwalk group of viruses and assess their roles as agents of diarrhea in a birth cohort in Peru. We will examine the importance of natural immunity to disease and the role of first exposure and protection from subsequent illness. About 180 newborns will be followed twice weekly to identify episodes of diarrhea and to collect stool specimens during these episodes and at monthly periods when the infant is free of symptoms. Stool specimens will be examined for viral agents using EIA's for rotavirus, enteric adenovirus and astroviruses, PCR for rotavirus characterization, Norwalk virus detection and astrovirus confirmation, and newer assays to detect novel viruses in a subset of specimens (eg. toroviruses, picobirnavirus). Sera specimens taken at four monthly intervals will be examined for antibody acquisition to astrovirus, Norwalk virus, and specific rotavirus outer capsid proteins to determine how often an infection with a specific agent is associated with an antibody response and to document for rotavirus and astrovirus whether antibody present prior to infection affects the occurrence or severity of symptoms upon reinfection. This study should improve our ability to understand the role of viral etiology in diarrheal disease in Peruvian children, and determine the potential role of protective immunity from first infections, an important consideration in vaccine development.